Kaganowicz, in U.S. Pat. No. 4,328,646, disclosed a method for preparing an abrasive silicon oxide (SiO.sub.x) coating on a plastic substrate, such as a patterned lapping disc, which includes the steps of subjecting silane and a gaseous, oxygen-containing compound to a glow discharge. The resulting abrasive SiO.sub.x coatings deposited on the substrate are suitable for lapping a hard material, such as diamond.
Abrasive coated discs of the type mentioned above have been successfully employed to micromachine diamond articles of micron and sub-micron dimensions, such as styli for high density information discs. In order to meet the stringent specifications of a high density information disc playback system, the diamond styli must be machined to a high degree of accuracy. It has been found that uniformity of quality and thickness of the SiO.sub.x coating on the stylus lapping disc is important to the successful manufacture of video disc styli.
Many commercially available glow discharge systems suitable for depositing SiO.sub.x coatings onto lapping disc substrates employ electrodes which are smaller in area than a typical stylus lapping disc (30.5 cm in diameter). To adapt to such a system, the lapping disc substrate is mounted parallel to the electrodes so that the disc can be rotated through the electrode area in order to coat the entire substrate surface with the product deposited during the glow discharge. Because it is known that coatings of different quality and thickness are deposited in different areas of a glow discharge system, magnets have been added to, but electrically insulated from, the electrodes. The magnets keep the glow in the electrode area, thereby providing somewhat better uniformity of the deposited coatings. However, the variations across a substrate with respect to uniformity of thickness and quality of SiO.sub.x coatings deposited on the lapping disc in this manner, although improved, are still less than desirable for the highly precise manufacture of the above described styli. Also, when using an electrode smaller than the substrate, if a higher power is sought to be used to promote a more rapid deposition, heat builds up in the system which warps the plastic substrates. Merely enlarging state of the art electrodes to be comparable in size to the lapping disc substrate does not enhance the uniformity of the deposit.
It should be stressed that the desired improvements in the uniformity of quality and thickness of a coating across the substrate should not be obtained at the expense of established standards for the production of stylus lapping discs, if possible. Factors such as power usage, SiO.sub.x precursor consumption and deposition rate are important economic considerations. Further, the temperature in the system should be minimized as a practical matter to avoid damaging the heat-sensitive plastic substrates used in the production of lapping discs.